This invention relates to a continuous method for coating an electrically conductive article. More particularly this invention is an improvement of a method for coating an electrically conductive article by electrodeposition wherein the article to be coated is used as a cathode and is coated by electrodeposition in an aqueous bath containing a basic resin at least partially neutralized with an acid compound and a synthetic resin in the form of fine powder.
There is known (U.S. Pat. No. 3,869,366 dated Mar. 4, 1975) a method for coating an electrically conductive article with a resinous material by electrodeposition which comprising immersing said article as the cathode in an aqueous bath containing, as essential components, (1) a water thinnable cationic binder resin which is a nitrogen atom-containing resin neutralized at least partially with an acid compound and (2) at least one non-ionic resin in the form of powder which is solid at the room temperature but can melt when heated to form a film, and applying a voltage between said cathode and an anticorrosive electrical conductor as an anode through said bath, so that said cationic binder resin and non-ionic resin powder are deposited on the surface of the article, the amount of the non-ionic resin being 100-5000 parts by weight per 100 parts of the binder resin, and the solid content in the aqueous bath being 10-20% by weight.
Such cationic electrodeposition system wherein a cationic binder resin is used is advantageous as compared with other conventional electrodeposition coating methods, for example, as follows:
(1) The Coulomb efficiency is higher. Therefore, the amount of consumption of electricity per unit amount of resin deposition is small.
(2) The amount of resin deposition time is large so that a coating film with sufficient thickness can be obtained in a very short period of time. Therefore, the production efficiency can be remarkably increased.
(3) The film thickness of about 100 microns or larger can be obtained. Since the film thickness is considerably larger than in a conventional method, the hurt concealability of the base of the article to be coated is high, and the feel, luster and properties of the coating film are excellent.
(4) Coating film properties as excellent as of a coating film by a conventional powder painting method will be able to be obtained.
In carrying out such cationic-type electrodeposition, the amount of the non-ionic synthetic resin powder to be dispersed in the aqueous bath is 100-5000 parts, preferably 100-1000 parts by weight per 100 parts of the cationic binder resin in the bath, and the solid content in the aqueous bath is 10-20% by weight. The bath is maintained at 20.degree.-30.degree. C. and a voltage is applied between the cathode (article to be coated) and anode to cause direct current flow through said bath for about 5-30 seconds. The voltage to be applied is generally about 100-600 V. The pH of the bath is 4.0-6.0.
For more details about the electrodeposition process wherein the cationic binder resin is used, reference may be made to U.S. Pat. No. 3,869,366. The present invention is directed to an improvement of such cationic type electrodeposition process.
Thus when such cationic type electrodeposition coating is carried out continuously, the respective components in the bath will not decrease at the same rate so that there is caused fluctuation in the bath composition. For example, with the progress of the electrodeposition, the basic resin (binder resin) and fine synthetic non-ionic resin powder will be consumed but the acid compound used to render the basic resin water-thinnable to prepare the binder resin will remain in the bath and will occupy a relatively larger proportion in the bath. Under such state, when a replenishing supply liquid having the same composition as that of the initial bath is added, the proportion of the acid compound relative to the basic resin in the bath will continuously increase and the concentration of the acid in the bath will accordingly be increased. Under such condition, the following disadvantages will be incurred.
(A) The precoated film (formed by the pretreatment) on the article will be damaged or the article will be rusted, so that the properties of the final coating film will be adversely affected.
(B) Gas will be abnormally generated by the electrolysis at the time of the electrodeposition and as a result thereof many gas pores will be formed in the wet coating film, so that pinholes will tend to be formed when the film is baked and the uniformity of the coating film will be impaired.
(C) At the time of the electrodeposition, there will be caused difference in the electrophoretic rate between the cationic synthetic resin and the fine synthetic non-ionic resin powder. In some cases, the electrophoretic rate of the fine synthetic resin powder will be lowered so that the cationic resin will be contained more in the deposited coating film. In such case the coating film will lose the excellent properties owned by the fine synthetic resin powder, impairing the desired properties of the film. In some other cases, on the contrary, the content of the fine synthetic resin powder will become undesirably higher in the coating film.
(D) The electric current flowing at the time of the electrodeposition will become large and the rate of the ineffective current will increase so that the above described high coulomb efficiency will be lost.
Therefore, an object of the present invention is to provide a method wherein the above described disadvantages during the continuous long time operation of the cationic type electrodeposition coating method are overcome and the balance of the components in the electrodeposition bath is properly maintained.
Other objects of the invention will be apparent from the following description.